1. Field of the Invention
The present invention relates to improvements in a magnetic electron lens. More particularly, the invention relates to improvements in a magnetic electron lens used to focus and enlarge a charged particle beam by the whole range of charged particle beam-applied equipment including electron beam instruments such as electron microscopes and micro-probe secondary ion mass-spectrometers.
2. Description of the Prior Art
Heretofore, electrostatic and magnetic electron lenses have been used to focus and enlarge a charged particle beam (electron beam or ion beam) by electron beam-applied equipment such as electron microscopes as well as by ion beam-applied equipment such as microprobe secondary ion mass-spectrometers. Of these types of equipment, high-magnification electron microscopes designed particularly to observe the structure of specimens on the atomic scale need to use an electron lens with a small amount of lens aberrations to ensure specimen images of high magnification and resolution. The lenses that meet the above requirement are primarily magnetic electron lenses having smaller absolute values of lens aberrations than electrostatic electron lenses.
Up to now, the prior art magnetic electron lens can only constitute a convex lens, not a concave lens. There has been no conventional technique for combining the convex and concave lenses to minimize the amount of aberrations, as is customary with optical lens systems. While the resolution of optical microscopes is on the order of the wavelengths of the light used, electron microscopes (typically those that use an electron beam accelerated at 100 kV) have a resolution of about 0.1 nm as opposed to the electron beam wavelength of about 4 pm. That is, the resolution of electron microscopes is not high enough when compared with the wavelength of the electron beam employed. The cause of limit lies mostly in the influences of such lens aberrations as spherical aberration and chromatic aberration, which are attribution of the electron lens itself. (As theoretically unavoidable effects, there are diffraction aberration and the interaction between electron beam and inner electrons of a specimen, which are small enough. Traditionally, it has been considered impossible to correct the effects of the above-mentioned lens aberrations. A new technology such as electron beam holography, has been developed originally to eliminate the effects of spherical aberration that is intrinsic to the objective lens of electron microscopes. The electron beam holography technology constitutes from recording of electron diffracted images (Fourier transformed images) with using a coherent electron beam and optical inverse Fourier transformation using a laser beam. The purpose of this technology is to minimize the effects of aberrations of the electron lens. Publications associated with this field include "Theory and Application of Electron Microscope I" (edited by Japanese Society of Electron Microscope, published by Maruzen Co., Ltd., Tokyo, October 1959) and "The Electron Microscope" (E. & F. N. Spon Limited, London, 1961).
Because prior art magnetic electron lenses can only constitute the convex lens arrangement as described, there has been no way of combining, as in optical lens systems, a convex lens with a concave lens to correct the aberration of the convex lens. It is for this reason that electron microscopes utilizing the prior art magnetic electron lens have failed to provide sufficiently high resolution. The same can be said for the whole range of charged particle beam-applied equipment employing a finely focused electron beam or ion beam.